In the operation of an incinerator, it is desirable to control the generation of products of incomplete combustion (PICs). One way of controlling PICs is to incinerate at a high temperature. However, during system upsets, the temperature may increase to the point where furnace damage results. Several attempts have been made to provide control over both temperature in the furnace and the release of products of incomplete combustion. Generally, past attempts have had to sacrifice the control of one while concentrating on the control of the other during an upset.
Furnaces burning wastes can often have a variable waste feed composition. The amount of BTU value of this waste can vary dramatically. Attempts to homogenize the feed material by blending and mixing have been only partially successful. This variability causes the required process control of the furnace to be very difficult. Furnace temperature must be kept above a permitted level to assure good waste destruction, but must be prevented from being so high that damage to the furnace could occur. Draft must be kept at a negative pressure so that gases and dust cannot escape into the atmosphere. Residence time of the gas is often required to be above a prescribed value to assure complete waste destruction. And sufficient levels of excess oxygen must be available to completely burn the waste material.
A furnace using air as the oxidant controls the furnace temperature by varying the fuel flow rate to the burners and simultaneously varying the air flow rate proportionately to the fuel flow. Excess oxygen is provided by adjusting the ratio of air to fuel, and from air infiltration into the furnace. If a large amount of BTU value is in the waste material, the furnace temperature will start to rise as the heat is released from the waste during incineration. The control system, sensing this rise will start to decrease the amount of fuel being sent to the burner. However, due to the thermal mass of the furnace and the time lag of the temperature control loop, the furnace temperature will overshoot significantly before any correction is made. The air flow rate will also be reduced accordingly, since it is on a proportional basis with fuel flow. This combination of events cause the excess air to be depleted rapidly, and PICs can be released. The temperature is actually prevented from reaching higher levels since there is not enough available oxygen to react with all the available fuel. An operator alerted to these events can manually increase the amount of air being fed to the furnace. However, he is restricted in the amount of air he can add, since adding air will reduce residence time and/or drive the furnace into a positive pressure condition, because for every part of oxygen added from the air, four more parts of nitrogen are also added. These constraints normally prevent the automation of this type of response.
Some incineration systems set the combustion air flow manually at the maximum level in order to buffer the fluctuations in heat release and oxygen demand, while adjusting the fuel flow in response to furnace temperature. Slow response and limited controllability of furnace conditions are the drawbacks to this approach.
Some incineration systems are controlled such that liquid waste flow is cut off when the oxygen level drops below a certain point. However, such a measure is drastic and as a result it tends to upset the smooth operation of the incinerator.
A furnace that utilizes oxygen enrichment can prevent some of the above limitations. Temperature control is the same as in the case of air combustion. However, the excess oxygen level can be controlled directly by varying the oxygen flow. The negative effects of varying the air flow caused by the large amounts of nitrogen in the air are reduced significantly by using pure oxygen. This method of control has been shown to be very effective in controlling excess oxygen levels, and thereby controlling releases of PICs. However, under certain furnace upset conditions, the oxygen injection control method experiences problems. When a depletion of the available oxygen in the furnace occurs due to a rapid release of fuel such as volatile organics from the waste material, the oxygen percent control system responds quickly and increases oxygen flow. This causes these organic materials to be burned in addition to the fuel already being added for temperature control. The dynamics of the furnace and the high thermal inertia of the system cause the temperature control loop to be tuned for slow response. Thus, for a period of time too much heat is being added to the furnace and temperature rises until the temperature control loop can correct the excursion. In addition, the combined oxygen demand due to the burning of waste material and fluid fuel may exceed the capacity of the oxygen supply system, leading to emission of PICs. Also, the combustion products generated may exceed the flue gas handling capacity. Therefore, it is desirable to find a solution to be able to react quickly to high heat release from the waste material, rather than waiting for the temperature loop to respond.
Accordingly, it is an object of this invention to provide a method for operating an incinerator wherein the requisite oxygen demand in the incinerator is satisfied, thus controlling PICs generation, while maintaining appropriate incinerator temperature control so as to avoid equipment damage and while maintaining flue gas volume within design limitations.